The hermetic glass-metal connection has its principal application in the field of the electronic or electrotechnical industries for constructing enclosing boxes and hermetic electric passages. The metal is employed for its conductive properties, while the glass performs the function of an insulator. The connection between the glass and the metal must be both sealed and strong. The seal permits protecting the electronic or electrotechnical components from the exterior environment, while the mechanical resistance of the connection is necessary for resisting mounting or utilization stresses. Such a connection is usually obtained by one or more passages of the materials in heat treating furnaces operating under atmospheres.
Three treatments may be distinguished in succession.
First treatment: decarburization/degassing of the metal so as to avoid the formation of bubbles at the interface in the subsequent treatments, which would otherwise have an adverse effect on the mechanical resistance of the connection. This treatment is carried out at a temperature on the order of 1000.degree. C. under an atmosphere which is decarburizing but reducing with respect to the metal, for example with a mixture of hydrogen and water vapour and, optionally, a complementary inert gas such as nitrogen. The decarburization of an alloy based on iron, nickel and cobalt was studied by M. R. Notis in an article entitled "Decarburization of an Iron-Nickel-Cobalt Glass Sealing Alloy" which appeared in the "Journal of the American Ceramic Society", volume 45, No. 9, Sept. 1962.
Second treatment: oxidation of the metal after decarburization/degassing so as to form a layer of oxide required for obtaining a chemical connection in the course of the third treatment. This oxidation is carried out at a high temperature under an atmosphere which is oxidizing with respect to the metal, for example with a mixture of water vapour and an inert gas such as nitrogen and, optionally, with a low content of hydrogen. The necessity of very precisely controlling the oxidizing power of the atmosphere was studied by R. P. Abendroth in an article entitled "Oxide formation and adherence on an Iron-Cobalt-Nickel Glass Sealing Alloy" which appeared in "Materials Research & Standards", Sept. 1965.
Third treatment: sealing of the assembly of glass and metal by fusion or softening of the glass so that it wets the decarburized and oxidized metal. The sealing is carried out at a temperature slightly lower than the decarburization/degassing temperature, under a controlled atmosphere.
The treatment atmospheres which are employed must have well-determined water vapour contents since these water vapour contents intervene both as such and in relation in a ratio with the hydrogen content so as to impart to the gas a decarburizing effect, an oxidizing effect, a reducing effect, a neutral effect.
Moist atmospheres for these glass-metal connections are at the present time obtained by a simple bubbling of the gas in a vessel containing water. However, this manner of proceeding has the drawbacks of imposing a dew point which is roughly equal to the ambient temperature, and fluctuations of the dew point by variation of the ambient temperature and of the water level in the bubbler. To obtain dew points higher than the ambient temperature, the water of the bubbler must then be heated and, if it is desired, on the contrary, to obtain dew points lower than the ambient temperature, the water of the bubbler must be cooled or the water-saturated gas must be diluted by a dry gas. Furthermore, to obtain a well-fixed dew point, it is at the present time necessary not only to regulate the temperature of the water of the bubbler, and therefore to employ thermally-insulated baths provided with thermostats, but also to regulate the water level and therefore to employ a level probe coupled with an automatic filling device. An arrangement of this type is costly, but moreover, notwithstanding all these improvements, different drawbacks still subsist:
On one hand, the value of the dew point produced is a function of the geometry of the bubbler (in particular of the water/gas exchange surface area), whence the obligation to determine by calibrating the conditions of operation of each bubbler (heating and dilution) so as to obtain the desired dew points.
On the other hand, in the case where it is desired to change from one dew point to a higher dew point, this change being achieved by heating the bath, presents the drawback of having a long response time.
Lastly, in the case where it is desired to change to a lower dew point, the cooling of the bath presents the same drawback of thermal inertia as above, while the dilution presents the drawback of a change in the total flow when changing from one dew point to the other.
These drawbacks may of course be avoided by a quasi-infinite extension of the water/gas exchange surface area by passing, for example, through a large volume of porous material as described in the article entitled "Advanced gas Moisturizing System for Ceramic processing applications" by F. W. Giacobbe, which appeared in "American Ceramic Society Bulletin", Volume 66, No. 10, 1987, and by conservation of the total flow by readjustment of the flows of moistened gas and dry gas.
However, such a moisturizing system still somewhat lacks flexibility. Its construction is very costly and the problem of thermal inertia when it is desired to change the temperature of the bath, has still not been solved.